South Korea decided to purchase 40 radar-evading warplanes for deployment from 2018-2021 for its restarted fighter procurement project on Friday, making Lockheed Martin’s F-35 warplane its sole option.

At a meeting of the Joint Chiefs of Staff Council, the military decided to buy 40 stealth aircraft first and then 20 additional warplanes later in consideration of shifts in the security environment and technological development.

Officials expect the stealth fighter to serve as a strong asymmetric deterrent asset against the unpredictable regime in Pyongyang.

“When the U.S. F-22 stealth fighter operated on the peninsula, there appeared to be commotion in the North trying to detect it,” a military official told media, declining to be named.

“It is also said that the North Korean leadership cuts down its outside activities when a radar-evading plane is nearby.”

Seoul seeks to purchase the F-35A, which is the fighter’s air force variant while the F-35B is the Marine Corps’ short takeoff and vertical landing variant and the F-35C the Navy’s carrier-based version.

In 2018, Korea is expected to procure the F-35A equipped with the final combat-capable software version of Block 3-F, which is to be completed in late 2016.

Analysts say the possibility remains that Seoul could purchase the remaining 20 aircraft from other companies such as Boeing and the European Aeronautic Defense and Space Company to gain technology for its separate “KFX” project to develop a home-built warplane.

The JCS council decided to turn the KFX project, which was initially a long-term project, into a mid-term one, and to quickly craft a basic system development plan, as the Air Force is expected to face a shortage of around 100 fighters in 2019 when almost all of the F-4s and F-5s will be decommissioned.

Sources presumed that the KFX development would finish around 2020, with deployment to begin in 2023.

It has been widely expected that Seoul will seek to purchase stealth jets as neighboring states such as China, Japan and Russia are pushing to procure their own stealth aircraft.

China is trying to develop fifth-generation aircraft such as the J-20 and J-31 while Japan has signed a contract with Lockheed Martin to purchase 42 F-35 jets. Russia is working on building its own T-50 stealth fighter with the goal of deploying it in 2016.

The single-seat, single-engine F-35 is an all-aspect stealth fighter still under development. Controversy has lingered over the U.S.-led multinational F-35 development program due to the discovery of defects, development delays and surging costs.